The present invention relates to a new and improved construction of a smoke detector.
Generally speaking, the smoke detector of the present development is of the type containing a radiation source operated in a pulsed mode and delivering corresponding source output pulses. A radiation receiver is provided externally of a direct radiation region of the radiation source and which, in the presence of smoke in the radiation region, is impinged by scattered radiation and delivers output signals. There is also provided an evaluation circuit containing switching elements which, when the number of source output pulses or signals exceeds a predetermined threshold value, transmit a signal to a toggle stage for outputting an alarm signal.
Such type of smoke detector has been disclosed to the art, for instance, in the European Patent application No. 80/1326 and the published European Patent application No. 14,779. A radiation source is controlled by a pulse transmitter which transmits briefly lasting radiation pulses. The radiation receiver receives the radiation which is scattered throughout certain scattered volumetric regions by the smoke emanating from a combustion process, but also receives radiation which is reflected by the walls of the smoke detector.
For compensation of a transmitter and receiver against the effects of aging and temperature there has already been proposed in U.S. Pat. No. 4,180,742, granted Dec. 25, 1979, in the case of a scattered light detector operated with light or radiation whose intensity is constant as a function of time and containing a second identical receiver cell, to measure and regulate the transmitted light of the transmitter. However, such is insufficient for compensating against all possible changes due to contamination.